So I'm a little rusty at this and I've really never had a high level priest. I'm about to change from aco to priest and almost reaching 99 INT. What stat can I focus on next? I know AGI can make me more missable and VIT gives me more HP. Don't know if DEX is even that important since I'l just be healing and supporting. Please give me some thoughts and options thanks!

Usually you want atleast 40 dex so you wont take too long to cast kyrie, magnificat or maybe even safety wall. If you are in mobby areas having a short cast time is quite useful. I usually ignore agi mostly and go for some vit, but this might be a question of taste.